Effect of residual stress on the fatigue performance of the surface of a ballised hole

Journal of Materials Processing Technology, 29 (1992) 301-309

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Elsevier

Effect of residual stress on the fatigue performance of the surface of a ballised hole M.O. Lai, A.Y.C. Nee and J.T. Oh Mechanical and Production Engineering Department, National University of Singapore, Kent Ridge, Singapore 0511 {Received May 29, 1991; accepted June 10, 1991 )

Industrial Summary Ballising, the process of forcing a precision-ground tungsten carbide ball through a slightly undersized pre-machined hole, refines the surface structure of the hole and renders a plastically deformed hole surface where protrusions generated by the drilling or the boring of the hole before ballising are displaced plastically to fill up depressions. Consequently, not only are compressive residual stresses induced on the surface due to the cold-working that the hole surface has experienced, but significant improvements in surface finish, roundness of the hole and dimensional tolerance are also achievable. The present study investigates the effect of the residual stress on the fatigue performance of a ballised hole. It has been found that the fatigue performance is dependent upon two factors, namely, the completeness of the ballised hole and the interference between the bore and the ball. The fatigue life was, expectedly, observed to increase with the increase in interference, but when the ballised hole was broken, the fatigue life decreased to below that of an unhallised specimen having approximately the same range of surface roughness. Residual stress studies using a fracture mechanics approach were conducted to evaluate the residual stress on the ballised hole surface. The result showed that when the hole is complete, compressive residual stress is induced at the hole surface, but when the hole is broken, the compressive stress is redistributed to give rise to a state of tensile stress at the hole surface. This finding is consistent with, and explains, the result of the fatigue tests.

1. Introduction

One of the most difficult machining problems involves the machining of holes, especially where precision in size, roundness and finish are important. Such a problem may be solved satisfactorily by employment of the ballising process. Ballising is a micro-finishing process where a precision ground ball of prescribed diameter is forced through a slightly smaller pre-machined hole, as shown in Fig. 1. The process is reputed to be fast and economical as it requires only simple and inexpensive tooling, especially when just a few specific hole sizes are desired. Ballising is basically a metal working process in which no metal is removed 0924-0136/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

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but in which a burnishing action (practically no rotation of the ball is involved) refines the surface structure of the hole, leaving it plastically deformed. Protrusions generated by the drilling or the boring of the hole prior to ballising are displaced plastically to fill up depressions. As a result, dramatic improvements in surface finish, roundness of the hole and dimensional tolerance are achieved [1,2 ]. At some optimum interference, a ten-fold improvement in surface finish has been reported [ 2 ]. This improvement, together with the compressive residual stresses on the surface due to the cold-working that the hole surface has experienced, are believed to improve the fatigue behaviour of the ballised material. 2. Background

Earlier works on ballising [3,4] using a Swedish medium-carbon Assab 760 steel have shown that, although the surface finish, roundness and dimensional tolerance of the hole have been improved dramatically, the fatigue performance of the ballised specimens has not been observed to follow a similar trend of improvement. On the contrary, the fatigue life appeared to have decreased in comparison to that of the unballised samples. The work of Ref. [4] has shown that the ballising process is inferior to that of polishing and reaming, in that among the three hole finishing processes, as can be seen from Fig. 2, the ballised samples produced the shortest fatigue lives for the same surface finish. On the other hand, the unpublished work of Panchal [5] with stainless steel showed that ballised holes yielded an approximately three-fold increase in fatigue life, see Fig. 3. However, comparison of the works of Refs. [3,4 ] and [5]

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has revealed that, besides difference of the materials, the test specimens employed were different also, the former having made use of a rectangular specimen that had been slit into two halves through the ballised hole so that the hole surface could be directly loaded under 3-point bending mode, whilst a tension specimen with a complete unslit ballised hole at the centre was employed in the latter work. It is believed therefore, that the difference in fatigue performance was due to differences in the residual stress distribution in the test samples. The purpose of the present study is therefore to investigate the effects of

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residual stress at the hole surface, due to different specimen geometries, on the fatigue life of the ballised hole.

3. R e s i d u a l s t r e s s e v a l u a t i o n

In the present study, a fracture-mechanics approach proposed by Kang et al. [6] was used to evaluate the residual stress distribution at the ballised hole surface. The basis of this approach, more fully documented in Ref. [7], considers that as the virtual forces approach zero, the displacement between the two points across a crack in a solid body subjected to external forces, as shown in Fig. 4, is a function of the stress intensity factor due to the external forces and the virtual forces. By replacing the external forces with the residual stress, the stress intensity factor due to a crack in a body with residual stress can be determined using the principle of superposition and the Buckner weight function. In order to apply the method to a plate with a central ballised hole surrounded by a region of residual stress field, the Buckner weight function for a plate with a crack emanating from a centre hole was determined. For the case of the ballised specimen that had been slit into half, the appropriate weight function was evaluated likewise. Finally, the relationship between the residual stress, the stress intensity factor and the displacement associated with the residual stress released by the introduction of a crack was developed. The displacements, monitored experimentally as the cracks were extended incrementally, were used to calculate the magnitude of the residual stress field around a ballised hole.

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4. Experimental procedures

The material used in the present investigation was Assab 760 with a typical chemical composition of: 0.50% C, 0.3% Si, 0.6% Mn, and 0.04% S (nearest equivalent: AISI 1050 or En 43). The material was used in the as-received unannealed condition, having an approximate hardness of 200 HB, tensile strength 640 MPa and 0.2% proof stress of 340 MPa. Rectangular specimens of 76 × 200 mm were cut from 19 mm thickness flatplate stock. After centre marking, a pilot hole of 16 mm diameter was drilled conventionally, the pilot hole then being enlarged carefully to the desired diameter on a lathe with a carbide insert boring bar. The hole was then ballised on an automatic ballising machine known as the AUTO-multi function press set with single-stroke one-way operation. A 19.046 mm diameter tungsten carbide ball of grade AFMA 25 was used in the ballising process, without the application of lubricant. After ballising, a crack in the form of a saw cut of length 2 mm in the direction transverse to the long axis of the plate specimen was then introduced at two diagonally opposite edges of the hole, as shown in Fig. 5. The displacement between points C and C', a separation of 60 mm as selected in the residual stress formulation, was measured to an accuracy of 0.5 pm using a travelling microscope. The two cracks were extended simultaneously in a step-wise manner at increments of 2 mm until the crack length reached 14 mm. The residual stress distribution around the ballised hole was then computed from the mea-

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306 sured displacement. The residual stress for a crack length beyond 14 mm was not studied, since the magnitude of the stress was believed to be small and since the accuracy of the method may be affected by the boundary conditions of the test sample. For the ballised specimens that had been slit into halves, similar procedures were carried out to monitor the displacement. 5. Results and d i s c u s s i o n

Figure 6 shows the typical computed residual stress distribution around a ballised hole where the specimen has not been slit into halves [7], i.e., for the case of a complete hole. The interference between the ball and the pre-machined bore used in Fig. 6 was 1%. It can be observed that the region immediately adjacent to the surface of the hole exhibits a residual stress that is compressive, the compressive nature of the residual stress manifesting itself in closure of the crack when the latter was introduced with a saw: a grip on the saw blade was felt clearly. The magnitude of this compressive stress decreases until it reaches a maximum tensile stress at a distance of about 3.5 mm from the surface of the hole. Thereafter, the tensile stress gradually decreases towards zero towards the edge of the plate. The features depicted in Fig. 6 are similar to those observed in a fastener hole that has been cold-worked with an expanding mandrel [8], although the magnitude is different due to the severity of the deformation induced. Residual stress that is compressive in nature is generally recognised as being favourable to fatigue resistance, since fatigue cracks find it much more difficult to propagate through a compressive stress field [9]. To ensure crack initiation and fatigue crack propagation in situations such as that of a ballised hole, a tensile stress of sufficient magnitude must be applied to offset the compressive o o_ ,~-

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residual stress that surrounds the hole. The result is that either a longer fatigue life is observed for specimens that are moderately loaded, or a higher load is required to fail a specimen within a moderate fatigue life. The existance of a region of compressive residual stress explains, therefore, the increase in fatigue life in Panchal's work [5 ], where test samples with completely uninterrupted ballised holes were tested. The results for the case where the test specimen with a ballised hole was slit into halves were distinctly different from those observed above. Here the residual stress is entirely tensile in nature, as can be seen in Fig. 7, which shows the residual stress field for the specimen with 5% interference: The stress distribution was dramatically altered from that of compression in the vicinity immediately adjacent the hole to one in which no compressive residual stress could be observed. The magnitude of the tensile stress, although it was not high, was clearly observed to decrease with decrease in interference. The slitting of the specimen has therefore not only released all the compressive residual stress due to the ballising process, but it has also caused the residual stress to redistribute such that a tensile stress has resulted. Fatigue life depends significantly on the condition and properties of the surface layer of a machined component: Modifications to the surface affect the length of the nucleation stage of the fatigue process significantly. Surface unevenness acts as a stress concentrator and shortens the length of the nucleation stage. Likewise, macroscopic tensile stresses in the surface layer also shorten the nucleation stage [10]. When the ballised hole is complete and uninterrupted, both the improvement in the surface finish and the induced compressive residual stress work together to render an increase in the fatigue life. However, when the ballised hole is slit, not only is the beneficial compressive residual stress absent, but it has now in effect become tensile such that i

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any micro-notches that appeared during the working or processing would tend to open up and hence lower the fatigue life [10]: this may be the case depicted in Fig. 7. The benefit derived from the improvement in surface finish appeared to be less than the adverse effect due to the tensile residual stress, therefore giving an overall shortening in fatigue life for the slit specimen. The results of the present study indicate that the ballising or cold-working of a hole can only improve the fatigue performance of the hole when the hole remains complete and uninterrupted. Once the hole is opened, for example by a crack propagating from outside, its fatigue resistance may be even lower than before it was cold-worked. 6. C o n c l u s i o n

From the results of the present investigation, the following conclusions can be made: ( 1 ) Using a fracture-mechanics approach, the residual stress of a completely uninterrupted ballised hole was found to be compressive at the immediate hole surface. The compressive residual stress decreased until it reached maximum tensile at a distance about 3.5 m m from the surface of the hole. Thereafter, the tensile stress gradually decreased towards zero towards the edge of the specimen. (2) The residual stress at the surface of a ballised hole that has been slit was observed to be tensile in nature. (3) The increase in fatigue life in the uninterrupted ballised hole and the decrease in fatigue life in the slit ballised hole were attributed respectively to the beneficial compressive residual stress and the adverse tensile residual stress.

References 1 2 3 4 5 6 7

A.Y.C. Nee and V.C. Venkatesh, Bore finishing - The ballisingprocess, J. Mech. Work. Tech nol., 6 (1982) 215-226. A.Y.C. Nee and V.C. Venkatesh, A study of the ballising process, Ann. CIRP, 30 (1) (1981) 505-508. M.O. Lai and A.Y.C. Nee, Fatigue resistance of ballised hole surface, 2nd Int. Conf. Fatigue and Stress, Imperial College, U.K., September 1988, pp. 13-18. M.O. Lai and A.Y.C. Nee, The effect of several finishing processes on the fatigue resistance of hole surfaces, J. Eng. Mater. Technol., 111 ( 1989 ) 71-73. A. Panchal, Strain distribution in ballising, Bachelor of Science degree project report, The Hatfield Polytechnic, UK, 1983. K.J. Kang, J.H. Song and Y.Y. Earmme, A method for the measurement of residual stresses using a fracture mechanics approach, J. Strain Anal. Eng. Des., 24(1 ) (1989) 23-30. M.O. Lai, J.T. Oh and A.Y.C. Nee, Application of fracture mechanics in residual stress measurement of cold-worked holes, Int. Conf. on Fracture of Engineering Materials and Structures, Singapore, August 1991, pp. 859-865.

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J.Y. Mann and G.S. Jost, Stress fields associated with interference fitted and cold-expanded holes, Met. Forum (Australas. Inst. Met.), 6 (1) (1983) 44-53. 9 J.A. Collins, Failure of Metals in Mechanical Design, Analysis, Prediction, Prevention, Wiley, New York, 1981. 10 A. Puskar and S.A. Golovin, Fatigue in Materials: Cumulative Damage Processes, Elsevier, Amsterdam, 1985.